Question

Question #425ea

Answer 1

The primary kinetic isotope effect does not affect the reaction mechanism; rather, it gives information on the reaction mechanism.

Explanation:

Kinetic Isotope Effect

The kinetic isotope effect (KIE) refers to the change in rate caused by an isotopic substation in a molecule.

Frequently, the KIE refers to the effect of substituting `"D"` for `"H"` (the `k_"H"//k_"D"` ratio).

The `"C-H"` and `"C-D"` vibrations are quantized per the condition

`E = (n + 1/2)hf`, where `n = 0, 1, 2, …`

At ordinary temperatures, most of the molecules are in the `n = 0` state.

We see the maximum KIE when a `"C-H"` bond is being broken in the transition state, because the vibrational mode disappears.

Since `E_0("H") > E_0("D")`, `E_"a""(H") < E_"a""(D)"` and `k_"H" > k_"d"`.

This is called a primary kinetic isotope effect.

The calculated value of `k_"H" // k_"d" ≈ 7`.

A secondary KIE occurs when the isotope is substituted at a position next to the bond being broken.

Usually, for a secondary KIE, `k_"H"//k_"D" < 1.5`.

Mechanistic information from KIEs

KIEs give useful information about the rate determining step in a mechanism.

Case 1

`"CH"_3"CH"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CH=CH"_2`

`"CH"_3"CD"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CD=CH"_2`

`k_"H"//k_"D" = 6.7`

This is consistent with an `"E2"` elimination in which the `"C-H/D"` bond is being broken in the rate determining step.

Case 2

`"CH"_3"CH"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CH=C"("CH"_3)_2`

`"CH"_3"CD"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CD= C"("CH"_3)_2`

`k_"H"//k_"D" = 1.4`

This is consistent with an `"E1"` elimination in which the `"C-H(D)"` bond is not being broken in the rate determining step.